Bug #19140907 DUPLICATES IN UNIQUE SECONDARY INDEX BECAUSE OF FIX OF BUG#68021

Problem:

When a unique secondary index is scanned for duplicate checking, gap locks
were not taken if the transaction had isolation level <= READ COMMITTED. 
This change was done while fixing Bug #16133801 UNEXPLAINABLE INNODB UNIQUE
INDEX LOCKS ON DELETE + INSERT WITH SAME VALUES (rb#2035). Because of this
the duplicate check logic failed, and resulted in duplicate values in unique
secondary index.

Solution:

When a unique secondary index is scanned for duplicate checking, gap locks
must be taken irrespective of the transaction isolation level.  This is
achieved by reverting rb#2035.

rb#5910 approved by Jimmy

storage/innobase/row/row0ins.c

@@ -1702,7 +1702,7 @@ row_ins_scan_sec_index_for_duplicate(
 	do {
 		const rec_t*		rec	= btr_pcur_get_rec(&pcur);
 		const buf_block_t*	block	= btr_pcur_get_block(&pcur);
-		ulint			lock_type;
+		const ulint		lock_type = LOCK_ORDINARY;
 
 		if (page_rec_is_infimum(rec)) {
 
@@ -1712,16 +1712,6 @@ row_ins_scan_sec_index_for_duplicate(
 		offsets = rec_get_offsets(rec, index, offsets,
 					  ULINT_UNDEFINED, &heap);
 
-		/* If the transaction isolation level is no stronger than
-		READ COMMITTED, then avoid gap locks. */
-		if (!page_rec_is_supremum(rec)
-		    && thr_get_trx(thr)->isolation_level
-					<= TRX_ISO_READ_COMMITTED) {
-			lock_type = LOCK_REC_NOT_GAP;
-		} else {
-			lock_type = LOCK_ORDINARY;
-		}
-
 		if (allow_duplicates) {
 
 			/* If the SQL-query will update or replace